One of the most used means of communication between instruments and computers in laboratory environment is the GPIB and the associated GPIB protocol. It has found its use in research laboratories and industrial production as well as in clinical applications. A typical setup utilizing GPIB comprises a number of measurement devices e.g. Digital Multimeters (DMM:s) supply devices such as power supplies, control and transport devices e.g. switches and stepping motors, and computers for controlling measurement processes and collecting and analyzing data. The GPIB connects these GPIB devices together and is used for transferring both device instructions (commands) and data.
The GPIB was developed during the 1970s by Hewlett-Packard. It was accepted as an open standard, ANSI/IEEE Std. 488-1978, by IEEE (The Institute of Electrical and Electronics Engineers, Inc.) in 1978. The latest version is defined in ANSI/IEEE Std 488.1-1987 and IEEE Std 488.2-1992. GPIB is a parallel communication protocol supporting up to 15 GPIB devices, including at least one GPIB controller, that can be connected to one bus and controlled by a host computer. Data is sent in parallel 8 bits (1 byte) at a time with a maximum data transfer rate of 1 Mbyte/s (8 Mbit/s). According to the standard, GPIB cables may be of any length up to 4 meters. However, long cables lead to significantly lower transfer rates. To achieve the maximum transfer rate interconnecting cable links should be as short as possible with an average cable length between devices being less than one meter and with a maximum of 15 m total length per bus.
During operation a user application program, typically developed in LabView or C++, residing in a host computer, controls the connected GPIB devices as well as collects and stores measurement data through a GPIB interface. In addition, the user application frequently processes the data and presents it in graphical form. The GPIB interface consists of a GPIB controller device (hardware) and a device driver (software). The GPIB control device is typically designed as a PCI or ISA expansion card that is put into a free slot in the host computer, GPIB controllers are also available as “plug-ins” adapted for external ports such as the USB port. The device driver provides an Application Program Interface (API) that enables the application programs to send and receive device data, and send bus and device instructions to the GPIB bus via the GPIB controller device by calling GPIB functions in the device driver.
Well-recognized problems with using GPIB arise from the above-mentioned inherent limitations on maximum total cable length and the maximum average length between devices and the fact that cables are needed. All pieces of equipment must be placed close together resulting in obvious drawbacks. It is often a requirement, from measurement constrains, to place parts of the equipment in close proximity to other parts or to the measurement object. For example a high accuracy current meter should preferably be placed close to the measurement object in order to keep the signal cables short. The limitations in the GPIB cubic length forces all the rest of the equipment to be placed nearby, which may be unwanted both of measurement aspect and by the aspects of convenient handling. In other applications part of the equipment need to be separated from the rest, for example in “glove boxes” or vacuum chambers or in clinical use. The may be impossible to arrange, and if possible, often with great difficulties and cumbersome handling as a result.
In addition the cables are rather bulky and stiff. In order to ensure the communication the cables need to be secured to the devices with screws. This, and the stiffness of the cables, makes the system inflexible and the GPIB contribute to the often massive entanglement of cables found in measurement setups. The lack flexibility is manifested in that it is cumbersome and time-consuming to change or replace a piece of equipment. Due to the inflexibility it is often impractical to use a device in more than one measurement setup, even if that device is not in constant use in that setup, adding to the cost of the measurement setups.
The above-described limitations of the GPIB often result in a setup arranged to satisfy the GPIB requirements and not in a way that is the best from a measurement or a handling perspective. Often found in laboratories are GPIB cables hanging in the air to make them as short as possible, such hanging cables being a potential hazard both for personnel and to the equipment. Importantly, certain applications cannot utilize the advantages of the GPIB due to its physical constrains and other means of communication have to be chosen.
Furthermore the cable, physically connecting the measurement devices, may cause grounding problems or transmit and/or pick up unwanted signals. This can be a severe problem in sensitive measurement application with low signal to noise ratio.
The problems arising from the use of cables for the GPIB communication can be summarized in: inflexibility; not possible to electrically separate different units; limitations in range, difficult and costly to construct inlets in for example glove boxes and potential hazards from hanging cables.
A common way to address the problems outlined above is to deliberately or undeliberately violate the constraints on total bus length and average cable length between GPIB devices, an approach that sometimes work but can introduce transmission errors that are unacceptable in for example industrial or clinical applications. Extenders and expanders that can relieve these constraints are commercially available, for example from National Instruments, GPIB extenders effectively replace one, or part of one, GPIB cable with a communication over a different medium, e.g. fiber optics while the GPIB expanders combines two busses into one bus of double size. The GPIB extenders and expanders address some of the described drawbacks of the GPIB, but not the lack of flexibility and the cumbersome handling. On the contrary, the introduction of more units and different means of communication often complicates the measurement setup.
Another approach to relieve the limitations of GPIB is represented by the product GPIB-ENET/100 by National Instruments. This device makes it possible to control GPIB devices through an Ethernet base TCP/IP network, for example a local area network (LAN), which typically is already present in most laboratories, offices and industries. If combined with a W-LAN (Wireless LAN) router the communication with the GPIB-ENET/100 can be made wireless. However, this kind of wireless GPIB interface is inherently costly due to its high degree of complexity and the components needed for its realization. Furthermore, it is space consuming in cases when several GPIB devices are to be provided with one wireless interfaces each.